The present invention relates generally to apparatus for gauging the fluid level in a container, and more particularly to probe transmitter apparatus flexible in multiple planes for determining the fluid level within the container as a function of electrical capacitance. The invention finds particular utility in the aerospace field, for measuring the level of oil in a tank or compartment of an aircraft engine.
Fluid level gauges for aircraft oil tanks must meet stringent structural and operational requirements. Thus, aircraft oil tanks may be arranged in irregular shapes. For example, the oil tank of an A-4 jet aircraft engine has a saddle-shape which sweeps back and wraps around the jet engine. Moreover, such oil tanks often are both elongated and congested with convoluted internal piping and other structures, such as support structure. Therefore, it is desirable to provide a fluid level probe that is capable of flexing in multiple planes to allow insertion and withdrawal for maintenance. Also, the piping and other internal structure is often composed of metal or other conductive materials. Thus, it is necessary to provide a flexible probe which will not electrically short-out when in contact with such conductive internal structure. Finally, aircraft engine oil tanks present a hostile environment, with temperature gradients varying widely over short periods of time (e.g., engine startup) and reaching highs of about 400.degree. F. Accordingly, a fluid level probe must be composed of materials that operate efficiently, reliably and dependably over a wide range of temperatures in a hostile environment.
In the past, in order to determine the level of oil in the tank of an aircraft engine, point level sensors have been provided. For example, a first point level sensor is located at a position in the tank to indicate an 80% fuel level and a second point level sensor is positioned in the tank at a 20% fuel level. However, such point level sensor arrangements have a drawback in that they monitor only discrete levels and may require multiple sensors, which can increase the complexity of the system and decrease its reliability. Also, a leak or sudden loss of oil may not be detected until virtually no oil is left, resulting in dangerous and costly engine burnout or failure.
Variable capacitance continuous detecting probes for determining the level of fluid in a tank or container are known. For example, U.S. Pat. No. 4,806,847 describes a sensor for sensing the level of oil or transmission fluid in a tank or vessel. The sensor has a probe, including a pair of electrodes forming a level detecting capacitor, adapted to extend in the vessel to detect the level of the liquid. A reference capacitor is included which has substantially the same capacitance as the level detecting capacitor when no dielectric liquid is present between the electrodes of the level detecting capacitor. Circuitry is provided for measuring the capacitance of each capacitor and for deriving from the capacitance measurements a signal proportional to the level of the dielectric liquid at the sensor. However, such variable capacitance continuous detecting probes have a drawback in that they utilize a rigid probe transmitter structure that cannot be inserted into irregularly shaped tanks. Also, when inserted into an engine oil tank, such probes may present a conductive surface that can electrically short-out on conductive internal structures.